Harsh Mathur

• Mathur, Harsh; Joshi, Jayant; Moe, Thore Espedal; Pereira, Tiago & Nagaraju, K. (2025). The Hα Line as a Probe of Chromospheric Magnetic Fields. Astrophysical Journal Letters. ISSN 2041-8205. 987(2). doi: 10.3847/2041-8213/ade8ea. Full text in Research Archive Show summary

  We explore the diagnostic potential of the H α line for probing the chromospheric magnetic field using a realistic 3D radiative magnetohydrodynamic (rMHD) model. The Stokes profiles of the H α line are synthesized through full 3D radiative transfer under the field-free approximation, alongside the Ca ii 8542 Å and Fe i 6173 Å lines for comparison. The line-of-sight (LOS) magnetic fields are inferred using the weak-field approximation for the H α and Ca ii 8542 Å lines, while the Fe i 6173 Å line is analyzed through Milne-Eddington inversion techniques. The comparison between the inferred LOS magnetic field maps and the magnetic fields in the rMHD model revealed that the H α line core primarily probes the chromospheric magnetic field at log τ 500 = −5.7, which corresponds to higher layers than the Ca ii 8542 Å line core, which is most sensitive to conditions at log τ 500 = −5.1. On average, the Stokes V profiles of the H α line core form 500 km higher than those of the Ca ii 8542 Å line core. The H α polarization signals persist after adding noise, and with noise at the level of 10 −3 I c , most simulated magnetic structures remain visible. These findings suggest that spectropolarimetric observations of the H α line can provide complementary insights into the stratification of the magnetic field at higher altitudes, especially when recorded simultaneously with widely used chromospheric diagnostics such as the Ca ii 8542 Å line.

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